Frederick William and brother Thomas both have an affidavit for their baptisms in 1875 and 1877 given by their older sister Esther Mathilda Roberts living at Amherst NS when signing documents in 1945. So this means that Esther was married to a Roberts at that time. In between 1877 and 1881 Peter Campbell deceased as on the 1881 census you could find his wife Eliza already married to Asa Howell and with them Esther 9 yrs old, F. William 5 and Thomas 3 all listed under the name Hoell.
On the 1891 Census this family is again in Welford always listed as Howell and Eliza had three more children with Asa Howell. Fred William is 15 yrs old and Thomas 13 always under the name Howell.
On the 1901 census the family is still there but this time Frederick and Thomas are under their right names Campbell. In the same house there is an Henry or Honory Roberts 24 yrs old married sailor (could be the husband to Esther )
Asa Howell died in 1903 aged 59 yrs old.
Hope this is of some help to you.